An optical detector generates a small, current pulse in response to an incident optical pulse. The optical pulse may be very brief, for example, a few tens of picoseconds. There is often a large parasitic capacitance associated with the detector. This capacitance prevents the current pulse from generating a voltage large enough to trigger digital circuitry. Some sort of amplifier or pulse detector is needed to convert the optical pulse into a voltage pulse large enough to trigger digital circuitry.
In some applications, the pulse detector is used to receive optical clock pulses and convert these into electrical clock pulses for clocking digital electronic circuitry. In such applications, the timing of the edge is most important. The delay that occurs between the optical pulse and the electrical clock pulse should be as independent as possible from variations in transistor process parameters, temperature, and supply voltage as any variation contributes to skew error in the generated clock pulse. It is particularly important that the delay of the pulse detector be independent of dynamic variations in supply voltage, so as to avoid jitter in the timing of the clock edge. Both skew and jitter can cause a reduction in the maximum operating frequency of the digital circuit clocked by the pulse detector.
The common way to amplify the small signal generated by an optical detector in a high-speed circuit is with a transimpedance amplifier. These are used to amplify the small current pulses in a linear response, which means that the output pulse shape is similar to the input pulse, but with magnified voltage or current. Light incident on a detector is usually limited to low power by the cost of the laser to provide this light, or by attenuation of the light through the transmission medium such as optical fibers, waveguides, or couplers, or both. The photo-current generated by light on the detector, coupled with the capacitance of the detector and other connected circuits is usually not sufficient to swing the voltage far enough to drive logic circuitry, approximately 1 V in current CMOS technology. Thus, the signal usually requires amplification to generate for useful signal levels in logical systems.
The silicon technology that is used to fabricate the nanometer devices employed in such an amplifier is typically not fast enough to follow the small pulses generated by the detector. In short, one cannot build silicon-based devices or transimpedance circuits that are fast enough to follow less than a few picosecond pulse of light or electrical current.